Step 1: Tools and stuff...

hammer, a good one because you have a few thousand nails..
set of pliers.
ruler, scissors and cutting knife
sewing string (I bought a 500 yard string and use less than third..
nails - "Head less" about 300 grams - almost 4000 pieces, for this project - depends on the size and picture.
broken pen, so you can insert the string and stretch it (you'll see...)
the photo that you want to make.
thick wooden board .cut to the size, I used MDF

Step 2: Photo.....

take a photo and make a "line art", you can find it on-line..
basically you can just take a made photo or latter..

than, using a raster program (I used - http://www.blockposters.com/) print your photo..
it's easy to use and you can find other Instractables that might help you.

trim the edge and place it on the board.. (Don't glue it !!!!!)

Step 3: Nail it !!

simple as that...

like an outer line, nail the edges, than lift the paper and make sure no small scraps are stuck on the board..

1. use only nail with head, throw the bad ones !!! (the small kind is called "headless" but most of them still have some..)
the string won't hold the nails with no head and it's a mess to replace them...

2. use a thick board and small nails, or else you will reach the other side, (like I did..) (so I placed something on the back and glued it,,,

3. nails are dirty, if you don't want finger marks on your board place sheets of paper on the sides.

Another idea (though I haven't tried it) - create a stippled version of the image using a voronoi diagram and place the nails at the stipple points; then run a travelling salesman algorithm to join the dots with a single continuous non-overlapping path which you can duplicate in string. Google for weighted voronoi stippling tsp art